heathy Posted April 2, 2005 Report Share Posted April 2, 2005 (edited) My 2004 sherco seems fine when it's idling but with a tiny touch of the throttle it goes straight to full revs (as though it is sucking in too much air from somewhere) It started doing this about a month ago and caused the bike to overheat and I have had to replace the cylinder and get the barrel re-chromed because of this, I also took the opportunity to change the main crankshaft bearings and also the seals as I thought air could be getting in through the crankshaft seals, I also replaced every gasket in the engine. Now that the bike is back together I still have the same problem of the bike going to full revs with a tiny touch of the throttle. I have done the following things. 1) Totally disassembled the carburettor and checked everything inside the carburettor for signs of wear or damage 2) Replaced the crankshaft seals 3) Replaced all gaskets 4) Replaced all seals 5) Checked pipe joining engine to carburettor for wear or damage 6) Checked pipe from carburettor to air box for wear or damage 7) Checked reeds and reed block for signs of wear or damage 8) Cleaned the air filter and air box 9) Made sure fuel mixture is correct 10) Checked the timing is set correctly 11) Set the gas screws on the carburettor correctly Has anyone ever had a problem like this before or can think of anything else I could try? Thanks Steven Edited April 2, 2005 by Heathy Quote Link to comment Share on other sites More sharing options...
coxy Posted April 2, 2005 Report Share Posted April 2, 2005 i dunno if this will help you but i had the same problem of it going to full revs on my motor cross bike, i found out after a short while of stripping parts and cleaning them etc that the throtle cable had frayed half way and was jamming open the throttle and also that the throttle had too much grease on the bars/throttle casing so it was also holding the throttle and not letting it move as it should Quote Link to comment Share on other sites More sharing options...
heathy Posted April 2, 2005 Author Report Share Posted April 2, 2005 Hi coxy, thanks for your reply, I have ran the bike with the airbox off so I could see the slide moving, it moves around 5mm from idle and goes on full revs. Its very strange, Could anything inside the carb cause this problem as I have closely inspected and/or changed every other part that could leak. Also the slide seems fine. Quote Link to comment Share on other sites More sharing options...
its a kinda magic Posted April 2, 2005 Report Share Posted April 2, 2005 ask one or trials buddys to see if it possible to swap carbs and then throttle cable/twist grip of his machine. you'l soon see then if it is either of these causing the problem! very cheap and easy process of elimination.... Quote Link to comment Share on other sites More sharing options...
jaylael Posted April 2, 2005 Report Share Posted April 2, 2005 An extremely lean condition can cause runaway idle speed. It sound like you have eliminated air leaks as a possible cause. If the bike starts up and idles sweet, it is getting sufficient fuel at idle, and does not have an air leak. During a runaway episode, place your hand over the air horn on the carb. thus stopping intake airflow and creating a massive vacum in the fuel jets, sucking through alot of fuel and possibly any obstructions, then when you remove your hand if it revs up again, the obstruction has not been removed from the main jet or needle jet orifice. This step is helpful as a diagnostic for two reasons. One is that if an engine continues to run with intake air blocked off, it is sucking massive air someplace else. In your case this is not likely. I am thinking the carb is the culprit also. Look very carefully at the jet needle in the slide. Verify the needle jet has not rotated around or been intalled backwards, and that the needle clip is attached correctly. Also the choke plunger may be stuck. Make sure fuel flow through the petcock and screen are sufficient and the float level is not mis adjusted to the overly lean side of things. Good luck and let us know what you find. Quote Link to comment Share on other sites More sharing options...
heathy Posted April 2, 2005 Author Report Share Posted April 2, 2005 Hi Jaylael and big_john, thanks for your replies. Jaylael, I have just tried everything you said, the engine cuts out the immediately when I put my hand over the air horn. The needle looks ok and I have tried all clip positions with no result, the choke seems to move fine and the fuel flow from the petcock seems normal, the floats are move freely and I have checked that they float in a cup of petrol. Big_john, the spring is strong and I have also stretched it slightly, when you blip the throttle it goes to full revs (way above what the top end used to be before this problem started) but its not as though the slide is sticking because as soon as you let go of the throttle it will go back to idiling. thanks again for your replies, I think I've got the sherco from hell. Quote Link to comment Share on other sites More sharing options...
nigel dabster Posted April 2, 2005 Report Share Posted April 2, 2005 (edited) See if it does it when you eliminate cable ie pull up with old cable 5" long. Is float level set at 18.5? Carb swap is a good idea. O ring good on carb top? Plastic clip and metal securing washer plate thingy all right? Edited April 2, 2005 by Nigel Dabster Quote Link to comment Share on other sites More sharing options...
nigel dabster Posted April 2, 2005 Report Share Posted April 2, 2005 Take it to white bros and get it sorted? Quote Link to comment Share on other sites More sharing options...
heathy Posted April 2, 2005 Author Report Share Posted April 2, 2005 (edited) I've even tried running it with the throttle disconnected and just using a screwdriver to move the slide but its still the same. looks like i'll have to take it to white brothers to pressure test it. Edited April 2, 2005 by Heathy Quote Link to comment Share on other sites More sharing options...
nickerlias Posted April 2, 2005 Report Share Posted April 2, 2005 set fire to it&buy a gas gas Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted April 2, 2005 Report Share Posted April 2, 2005 Chech for air leaks at the head and cylinder base? Quote Link to comment Share on other sites More sharing options...
heathy Posted April 2, 2005 Author Report Share Posted April 2, 2005 I had a gas gas and the gearbox fell to bits! I have replaced the base gasket and the two orings in the head, I also coated them lightly with grease to ensure there was a ood seal, then I sprayed WD40 all over the joins while the bike was running to see if there were any leaks. I think its going to have to go to the shop because I've tried everything I can think of. Quote Link to comment Share on other sites More sharing options...
jaylael Posted April 2, 2005 Report Share Posted April 2, 2005 Steven: If you have given up the fight, disregard this advice, but I had another thought. Try propane enrichment during the runaway. I use a propane torch with a bit of rubber hose like fuel line attached to the propane outlet. When she takes off screaming, meter in some propane to richen it up until normal revs are obtained. The fact that it revs above what it would with normal wide open throttle indicates super lean condition. Is it possible the pilot jet is switched around with the choke jet? I'm not sure this is possible, but it's just a thought. It should have about a #38 pilot jet. What size are the jets in it now? Should be at least a #110 main but maybe going bigger would help this. Partially choking the air horn with two fingers should richen it up also and thusly slow it down. If you have thrown in the towel, I sure don't blame you. It sounds like a real tough problem. Jay Quote Link to comment Share on other sites More sharing options...
heathy Posted April 2, 2005 Author Report Share Posted April 2, 2005 The pilot jet is #33 and the main is #124 I really can't think of anything else that could be wrong with it. I'll explain how the problem started if that helps: It was running fine, then I took it out one day and it was normal for about 15 minutes but then I noticed the top end revs were much higher than usual, the top end gradually got worse until after about an hour it siezed due to the high high top end and ruined the piston and barrel. Now that I have rebuilt it the problem is even worse and it simply doesnt have a bottom rev range anymore. It revs seriosley high now eg. like many riders I used to open the throttle fully now and again to clean the engine out, it revs a lot higher now with a tiny touch of the throttle than when the throttle was fully open before. Because of the way it happened it seems like something worn very fast but I just don't know what it is. thanks again Steven Quote Link to comment Share on other sites More sharing options...
jaylael Posted April 3, 2005 Report Share Posted April 3, 2005 O.K. perhaps you have covered this already, but you just provided a substantial clue which was not stated before. The fact that it has completely lost it's idle is a very significant point. I suspect it is also hard to start then. The vacum leak has got to be pretty dang big! The only thing that leaks this bad is a torn carb to reed plate manifold. Try removing the carb. and push around with your finger to find out if the intake boot is torn. I'll wager it has a pretty big rip in it. Jay Quote Link to comment Share on other sites More sharing options...
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